In the design and manufacture of cartridge type electrical fuses of the type described among the generally sought objectives are to provide a fuse with a taut, centered fuse filament soldered to the end caps in the most effective and economical manner, with a minimum size for a given specific fuse rating, and minimum explosion risk during blowout involving high short circuit currents at high voltage which produce high energy arcs which can explode the fuse housing. With regard to the explosion risk, in very small fuses a relatively short arc can reach the end caps of the fuse and create an explosion hazard more readily than with larger sized fuses.
One of the most common and simplest, but least reliable, ways to make a miniature cylindrical cartridge fuse is to position the fuse filament diagonally disposed across the length of the cylindrical housing and captively secure the ends of the fuse filament between the outer ends of the fuse housing and the end caps. The physical and electrical attachment of the fuse wire ends to the end caps and the end caps to the housing are obtained by melting a solder pellet placed in each end cap. The solder is generally drawn by capillary action into the small clearance space between the end caps and housing to anchor and seal the end caps.
The blowout characteristics of such fuses tend to vary undesired degree from fuse to fuse because, as the operating currents slowly rises to the melting temperature of the fuse filament, the resulting expansion of the fuse filament can cause portions thereof which are near but spaced from the housing walls to sag and touch portions of the walls of the housing, which modifies the desired blowing characteristics of the fuse due to the heat sinking effect of the housing walls. The degree to which each fuse filament sags and the degree to which the fuse blowing characteristics of each fuse is modified can vary substantially from fuse to fuse. Thus, in the manufacturing process a delicate balance must be struck between the necessity for applying enough tension to the fuse filament during the attachment process to minimize sag, without over-stressing of the fuse filament, which can produce undesired weakening of the fuse or the stretching thereof which also modifies the fuse blowing characteristics.
For the above reasons, it is generally recognized that the better approach for making miniature fuses is to use a fuse design with a fuse filament centered in the fuse housing. However, such a disposition of the fuse filament, while inherently more reliable than the diagonally extending fuse filament as described, is more difficult to assembly and as in the case of fuses with diagonal fuse filaments the tension in the fuse filaments thereof is not readily closely controlled. In one process of fuse assembly having some aspects used also in the present invention, the fuse housing has end caps with centered holes therein, and a fuse filament carried by an insertion pin is passed through the fuse housing and the centered openings of the end caps. An operator then solders the fuse filament to the outer surfaces of the end caps with the fuse filament under manually applied tension. Since it is difficult for a person to accurately control the degree of tension applied to the fuse filament, in some cases inadequate tension and in the other cases excessive tension was applied which either broke the filament or unduly stretched the same, so as to undesirably modify the fuse blowing characteristics thereof.